Reproduction of sediment deposition and prediction of 137Cs concentration in the major urban rivers of Tokyo

Abstract

Radioactive caesium- 137 (¹³⁷Cs) can be used as a tracer to infer sediment dynamics due not only to its long radioactive half-life but also its affinity for fine sediment. A novel advanced interpolation assessment was conducted to examine radionuclide activity in terraced land covered with volcanic ash soil in Tokyo, Japan, which had a time-dependent input function and incorporated the effects of mixed-sediment particle dynamic behaviour on radioactive decay. In addition, transport parameters derived from Chernobyl measurements were applied as predictors of the long-term contamination of the cardinal urban rivers by the fallout from the Tokyo Electric Power Fukushima Daiichi Nuclear Power Plant (FDNPP) accident in 2011. The behaviour of suspended sediment substances, incorporating the effects of deposition and pickup, was assessed using a mixed-sediment particle dynamics model. The concentrations of ¹³⁷Cs adsorbed on fine sediment particles of each size fraction were determined. Removal of ¹³⁷Cs from the cardinal urban river channel had significant effects on both long-term decline, including extreme flash flood events, and the dynamic and time-dependent behaviours of interspersed ¹³⁷Cs and sediment activity. A novel advanced interpolation assessment method was used to examine radionuclide activity in terraced land covered with volcanic ash soil in Tokyo, Japan. The assessment procedure has a time-dependent input function and incorporates the effects of mixed-sediment particle dynamics on this time dependence. The results indicated that sediment and ¹³⁷Cs concentrations could decline more rapidly than observed in the Fukushima and Chernobyl regions. This rate of decrease depended on terraces covered with volcanic ash soil, which incorporated the effects of fine sediment behaviour for particle adsorption. In addition, comparatively large impacts were observed during extreme flash flooding events, which were associated with the land cover of the major urban river catchments in Tokyo. This work provides a new perspective for understanding ¹³⁷Cs behaviour associated with reproduction of sediment deposition and prediction of ¹³⁷Cs concentration in the major urban rivers of Tokyo, incorporating the effects of baseline ¹³⁷Cs behaviour with the impact of sediment particle adsorption in a volcanic ash soil-covered terrace.